Tin mill products include tinplate, electrolytic chromium coated steel (ECCS, also referred to as tin free steel or TFS), and blackplate, the uncoated steel. Packaging steels are normally provided as tinplate or as TFS, onto which an organic coating can be applied. In the field of packaging steels there is a growing incentive to reduce the amount of tin used for tinplate due to the increasing cost of the raw materials and resource depletion and reduction of the carbon footprint. The production of TFS currently involves the use of hexavalent chromium, which is a hazardous substance that is potentially harmful to the environment and constitutes a risk in terms of worker safety. There is therefore a need to develop alternative metal coatings that are able to replace conventional tinplate and TFS, without the need to resort to the use of hexavalent chromium and minimising, or even eliminating, the use of tin.
Packaging steel is generally provided in single and double reduced tin mill products. Single Reduced (SR) product is cold rolled directly to the finished gauge, then recrystallisation annealed. Recrystallisation is brought about by continuous annealing or batch annealing the cold rolled material. After annealing the material is usually temper rolled, typically applying a thickness reduction of 1-2%, to improve material properties. Double Reduced (DR) product is given a first cold reduction to reach an intermediate gauge, recrystallisation annealed and then given another cold reduction to the final gauge. The resulting DR product is stiffer, harder, and stronger than SR, allowing customers to utilise lighter gauge steel in their application. These uncoated, cold rolled, recrystallisation annealed SR and DR packaging steels are referred to as blackplate.
Tinplate is characterised by its excellent corrosion resistance and weldability. TFS typically excels in adhesion to organic coatings and retention of coating integrity at temperatures exceeding the melting point of tin. Tinplate is supplied within a range of coating weights, normally between 1.0 and 11.2 g/m2, which are usually applied by electrolytic deposition. Electrolytic Chromium Coated Steel (ECCS) or Tin Free Steel (TFS) consists of a blackplate product which has been coated with a metal chromium layer overlaid with a film of chromium oxide, both applied by electrolytic deposition. TFS can also be supplied within a range of coating weights for both the metal and chromium oxide coating, typically ranging between 20-110 and 2-20 mg/m2, respectively. Both tinplate and TFS can be delivered with equal coating specification for both sides of the steel strip, or with different coating weights per side, the latter being referred to as differential coated strip. Alternative metal coatings based on low amounts of tin, to replace conventional tinplate and TFS, should be able to match the characteristic product performance properties required for each substitution.
Reduction of the tin coating weight of conventionally produced tinplate, involving flowmelting of the electrodeposited tin coating, to below approximately 1 g/m2 leads to a deterioration of product performance in terms of corrosion resistance and compression of the welding range. This observation has resulted in alternative product compositions and process routes that are able to retain tinplate product performance, while reducing the tin coating weights applied. Examples include the application of thin nickel coatings (e.g. nickel coating thickness between 10-20 mg/m2) prior to tinplating to ensure retention of corrosion resistance and welding range at tin coating weights below 1 g/m2. However, due to the presence of a layer of passivated, unalloyed, free tin close to the outer surface of the product these materials are unsuitable to replace TFS as the adhesion to organic coatings and retention of coating integrity at temperatures exceeding the melting point of tin is insufficient.